The conventional transformers of a winding type and a lamination type are well-known as parts of an electronic circuit.
The winding type transformer comprises the primary and secondary windings on a ferrite core, which is hard to miniaturize. Since terminal treatment on the lead wire is difficult, the transformer is hard to mass-produce, whereby it is difficult to eliminate lead wires or to be chipped. Moreover, in order to adjust an inductance value, the number of turns of each winding must be changed and must be an integer, whereby it is difficult to obtain a desired inductance value.
On the contrary, the lamination type transformer, which enables miniaturization and chipping, has recently widely been used.
One conventional example of such a lamination type transformer is a laminated transformer 30 (FIG. 5) known as the through-hole system. The laminated transformer 30 comprises a plurality (seven in FIG. 5) of ferrite sheets 31 to 37 laminated in order and baked to be integral, each ferrite sheet including a conductor pattern or a through hole formed as follows. At first, on the surface of the ferrite sheet 31 is formed a J-shaped conductor pattern 31a extending at one end to the outer edge of the sheet 31. On the surface of the ferrite sheet 32 is formed a U-shaped conductor pattern 32a and at one end thereof is formed a through hole 32b perforating the ferrite sheet 32. On the surface of the ferrite sheet 33 is formed a J-shaped conductor pattern 33a extending at one end to the outer edge of the sheet 33 and at the other end of the conductor pattern 33a is formed a through hole 33b perforating the sheet 33.
On the surface of the ferrite sheet 34 is formed a J-shaped conductor pattern 34a extending at one end to the outer edge of the sheet 34. On the surface of the ferrite sheet 35 is formed a U-shaped conductor pattern 35a and at one end thereof is formed a through-hole 35b perforating the sheet 35. On the surface of the ferrite sheet 36 is formed a J-shaped conductor pattern 36a extending at one end to the outer edge of the sheet 36 and at the other end of the conductive pattern 36a is formed a through hole 36b. The ferrite sheet 37 does not have a conductor pattern or through hole.
The ferrite sheets 31 through 37 thus constructed are sequentially laminated and baked so that the conductor patterns 31a and 32a are electrically connected through the through hole 32b, and those 32a and 33a, through the through hole 33b. A primary coil is provided between one end of the conductor pattern 31a and one end of the conductor pattern 33a. The conductor patterns 34a and 35a are electrically connected through the through hole 35b, and those 35a and 36a, through the through hole 36b. A secondary coil is provided between one end of the conductor pattern 34a and one end of the conductor pattern 36a.
Another example of the conventional laminated transformer is called the printing system one as shown in FIG. 6.
Such a laminated transformer at first is provided on a burnable ferrite sheet 45 with a first conductor pattern 46 of about a half turn of the winding (FIG. 6a). Next, a second ferrite layer 47 is printed on the first conductor pattern in a manner in which one end portion of the first conductor pattern 46 remains exposed (FIG.6b). And, on the second ferrite layer 47, a second conductor pattern 48 is formed of about a half turn of the winding and is connected to one end of the first conductor pattern 46 (FIG. 6c). Such ferrite layers and conductor patterns are formed foregoing order and are baked to form an integral laminated transformer.
In the through hole system laminated transformer 30, the conductor patterns 31a and 32a constituting the primary coil are vertically opposite each other with the ferrite sheet 32 disposed therebetween, while patterns 32a and 33a are vertically opposite each other with the ferrite sheet 33 disposed therebetween. Similarly, the conductor patterns 34a and 35a constituting the secondary coil are vertically opposite to each other with the ferrite sheet 35 disposed therebetween while patterns 35a and 36a are similarly disposed with the ferrite sheet 36 therebetween. Hence, capacitors are constituted between the conductor patterns 31a and 32a, 32a and 33a, 34a and 35a, and 35a and 36a, thereby facilitating an increase in the self distributed capacity which promotes self-resonance, whereby the laminated transformer having such construction has not been suitable for use in a high frequency circuit. When the through hole 32b shifts from the conductor pattern 31a, or the through hole 33b shifts from the conductor pattern 32a, or the through hole 35b shifts from the conductor pattern 34a, or the through hole 36b shifts from the conductor pattern 35a during the lamination of ferrite sheets 31 through 37, the continuous winding cannot be formed so as to occasionally cause a problem such as disconnection, thereby lowering the reliability.
On the other hand, the laminated transformer of the printing system, similar to the through hole system, has the capacitor defined between the respective conductor patterns, thereby being defective in that this laminated transformer is not suitable for use in a high frequency circuit.
The manufacture of the through hole system laminated transformer requires much time to form the through holes and to connect the ferrite sheets through the through holes, while, the manufacture of the printing system requires much time to superpose the ferrite layers and conductor patterns. Hence, the laminated transformers of both the aforesaid systems involve a problem in that their manufacturing processes are complicated and require much time.